Water-cooled stator bars for electrical generators are comprised of a plurality of small rectangular solid and hollow copper strands which are brazed to one another within the stator bar and brazed to the interior walls of an end fitting. The brazing material typically used is a copper phosphorous alloy. The end fitting serves as both an electrical and a hydraulic connection for the stator bar. The end fitting typically includes an enclosed chamber for ingress or egress of stator bar cooling liquid, typically deionized water. Thus, the end fitting provides a hollow chamber which serves to manifold the water passages of the strands. Another opening of the end fitting receives the ends of the strands of the stator bar, the fitting and peripherally outermost copper strands of the stator bar being brazed to one another. Over time, leaks have variously developed about the connection between the stator bar ends and the stator bar end fitting as well as between adjacent strands. The end fitting-to-strand and strand-to-strand joints have the highest potential for causing damage should a leak occur because a leak at this location will flow directly into the ground wall insulation of the winding. The result is the degradation of the dielectric strength of the insulation which will eventually lead to failure. It is believed, based on leak analysis results, that the leak mechanism is due to a corrosion process which initiates in the braze alloy at the interior surface of the braze joint. Stagnant water in contact with the braze alloy and the copper strands is believed to cause corrosion and consequent leakage.
Field repair of leaks through the stator bar end connections has only been moderately successful. Typically, a leak site is identified by external visual examination methods where the strands enter the end fitting. External visual leak detection, however, can only indirectly identify the leak site because the external evidence of the leak may be located a considerable distance from the actual leak path. This is particularly true if the leak is occurring between the adjacent strands of the stator bar. Once a leak is identified in this manner, a vacuum is drawn inside the fitting and stator bar. An anaerobic cement is then applied externally around the suspected leak sites, the vacuum suction drawing the cement inwardly into the leak path. However, it has been found that this repair method is not 100% effective in repairing the leak and is therefore considered only a temporary repair. Another repair option, as described and illustrated in pending application Ser. No. 08/446,158 and of common assignee herewith, utilizes syringes and visual equipment to inject epoxy into the leak path. This process is limited by the space constraints of the hardware infringing on the syringe and visual equipment while on its way to the leak path.
Another alternative repair method, based on a method of original manufacture of the fittings dating to 1966 and used by the common assignee herewith, is the complete removal of the existing (original) end fitting of the stator bar so that the strands can be cleaned. After cleaning, a ring or sleeve is brazed onto the exterior surface of the strand bundle. As part of the repair, the joints between the strands as well as the joints between the strands and the ring can be brazed to fill any voids or water pathways. Finally, the ring and strand bundle can be inserted into a replacement fitting with the replacement fitting then brazed to the ring. This, however, is not a preferred repair method and was discontinued as an original construction method many years ago. This particular repair method is time consuming as it requires complete removal of the existing fitting. Another disadvantage is that the removal of the existing fitting in its entirely may adversely affect the integrity of the existing braze in the strand bundle. Also, and most importantly, this repair method involves a two part brazing process. The first braze is applied to the strand bundle and the ring which compresses and surrounds the strand bundle. The second braze is applied between the exterior surface of the ring and the interior of the replacement fitting.
Frequently, the inability to repair leakage through the stator bar end connections forces generator owners to replace the leaking bars or the entire stator winding to eliminate the leaks. An in-kind replacement with new components is not only expensive but requires significant generator downtime of approximately three to five weeks.